Method for producing a uniform metallic coating on wire



Aug. 11, 1970 L. J. BAXTER 3,523,815

METHOD FOR pnonucme A UNIFORM METALLIC COATING ON WIRE Filed Jan. 2,1968 INVENTOR/S LAwsm/J BAXTE/Q,

m $54M, 9%, am/ an ATTORNEYS United States Patent 3,523,815 METHOD FORPRODUCING A UNIFORM METALLIC COATING ON WIRE Lawson J. Baxter, Raytown,Mo., assignor to Armco Steel Corporation, Middletown, Ohio, acorporation of Ohio Filed Jan. 2, 1968, Ser. No. 695,097 Int. Cl. B05c11/02; C23c 1/08 US. Cl. 117-102 9 Claims ABSTRACT OF THE DISCLOSUREBACKGROUND OF THE INVENTION This invention relates to the continuous hotdip coating of wire with any of the conventional coating metals, such aszinc and its alloys, aluminum and its alloys, terne, and the like. Theinvention has great and particular utility in the coating of iron orsteel wire with aluminum, and the embodiment of the inventionspecifically described will relate to this field. It should beunderstood, however, that no limitations are to be inferred or impliedthereby.

More specifically, this invention relates to a continuous processwherein the wire passes upwardly through the bath of molten coatingmetal in a substantially vertical path of travel. Such a vertical modeof operation has the obvious advantage that during the periodimmediately after emerging from the coating bath, the force of gravityacting on the still liquid metallic coating does not tend to destroyconcentricity. However, and notwithstanding this great advantage, theart has had great problems in the production of intermediate and heavyweight metallic coatings using such a vertical process.

For example, one of the earliest of the various hot dip procedures waswhat might be called the free exit method in which the wire emergedvertically from a molten metal bath which was covered with a layer offlux. In a later version of this method, the flux was replaced with anon-oxidizing gas. The thickness of the coating produced was dependentalmost exclusively on wire speed, and hence it was possible to produce awide range of coating thicknesses. However, the coating applied by thisfree exit method was very rough, and is considered unacceptable for manycommercial applications. These problems are greatly exaggerated in thecase of an aluminum coating metal, because of the tendency of this metalto form a tough, gummy oxide which is pulled onto the moving wire andforms a highly irregular coated surface.

U.S. Pats. 2,914,423 and 3,060,889, both in the name of Earle L. Knapp,are directed in part to a method and apparatus for improving the surfacecharacteristic of a metallic coated wire. Both of these referencesinclude the provision of an exit die through which the wire passes afteremerging from the metallic coating bath. This exit die is completelyabove the normal level of the coating metal in the bath so that themoving wire Will pull up the molten coating metal to the die forming anoxide sock, while the substantially pure metal under the oxide layer iswithdrawn as a coating on the wire. While this method is satisfactoryfor the production of light coating weights,

3,523,815 Patented Aug. 11, 1970 it has been impossible to commerciallyproduce with any degree of consistency an intermediate weight coating.

One commercial application of intermediate weight coatings is in theproduction of aluminum coated wire for chain link fences. Such wire, tomeet the manufacturers specification, must have a minimum of .40 ounceper square foot of wire surface. Because of the cost of the coatingmaterial used, it is desirable to stay as close to this minimum level aspossible. In addition, the manufacturer also specifies minimum tensilestrength requirements, which are equal to the breaking load of thecoated wire divided by its cross sectional area. Thus, the thickercoatings make it more diflicult to meet the minimum tensile strengthrequirements. Keeping the foregoing comments in mind, it is thereforethe principal object of this invention to provide a method and apparatusfor uniformly and consistently applying intermediate weight coatings(i.e. on the order of .50 ounce per square foot) to a steel wire.

SUMMARY OF THE INVENTION Briefly considered, this invention contemplatesthat the exit die through which the wire is vertically withdrawn fromthe coating bath be substantially submerged in the molten coating metal.As the wire enveloped by a still liquid coating emerges from the exitdie, a non-oxidizing gas is blown against it at low pressure.

While many factors obviously influence the weight of coating materialapplied to the wire including such factors as speed of wire passage,temperature of the Wire prior to immersion, temperature of the coatingbath, diameter of the wire, amount of clearance between the surface ofthe wire and the exit die, and possibly others. Applicant has discoveredthat the primary factor in terms of controlling coating Weights withinthe limits of this application is the relative level between the topsurface of the exit die and the level of molten metal in the coatingbath. That is, the level of molten coating metal must be substantiallyat or just below the top surface of the exit die. If the molten metallevel gets much above the upper die surface, the rough coatingcharacteristic of the free exit method described above will result.Also, if the bath level gets below the upper die surface by more than apredetermined amount, the consistently maintainable coating weightswould be too light.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross sectional view showing acoating compartment according to this invention.

FIG. 2 is an end elevational view of the apparatus shown in FIG. 1 asseen from the right.

FIG. 3 is a top plan view of the apparatus of FIG. 1.

FIG. 4 is a perspective view of the die element.

DESCRIPTION OF THE PREFERRED EMBODIMENT In general, the hot dip coatingof steel wire includes the steps of thoroughly cleaning or otherwisepreparing the surface of the wire for the reception of a molten metalcoating. While these preparatory steps do not per se form a part of thisinvention, it will be understood that the invention contemplates such apretreatment prior to the time the wire arrives at the molten coatingbath.

Exemplary preparatory procedures now in commercial use are described indetail in Sendzimir Pats. 2,110,893, 2,136,957, and 2,197,622. Ingeneral, these patents contemplate that the wire is cleaned of oils,greases, and the like by passing it through an oxidizing furnace whereincarbonaceous foreign matters are burned from the surface of the strip,and a very thin, controlled oxide coating is formed thereon. The Wire isthen subjected to a heat 3 treatment in a reducing atmosphere whereinthe previously formed oxide layer is removed. Finally, withoutre-exposing the wire to atmosphere, it is passed directly into a bath ofmolten coating metal.

Instead of the described treatment in an oxidizing furnace, alkali orother chemical cleaning involving wetting and drying of the stripsurface, or even abrasive treatment may be used, so long as the surfaceof the wire is sufiiciently cleaned that an extremely rapid and thoroughwetting by the molten metal itself takes place in the coating bath.

Turning now to FIG. 1, one form of apparatus for practicing theteachings of this invention has been shown. The wire to be coated isindicated at 10, and as already indicated, it is to be assumed that thesurface of the wire has been adequately prepared to make it receptive tothe molten coating metal. The tube 12 is a schematic representation ofthe hood providing a protective atmosphere for the wire between the timeit leaves the reducing furnace and the time it enters the coating bath.It will be understood that this tube Will be supplied by conventionalmeans with a suitable protective atmosphere.

The bath of molten coating metal will be maintained within the bodyportion of the apparatus indicated generally at 14. As seen in thedrawings, the body portion is provided with the substantially verticalpassage 16, and with the passage 18 which is arranged substantiallynormal to the passage 16. As will be explained in more detail presently,the intersection of these two passages defines the bath of moltencoating metal.

Snugly maintained within the vertical passage 16 is the die elementindicated generally at 20, and shown in perspective in FIG. 4. Referringto that figure at this time, it will be seen that the die element isprovided with a horizontal passage 22, and with an intersecting verticalpassage indicated generally at 24. The passage 24 includes the lower andupper passages respectively indicated at 24a and 24b. As shown in FIG.1, the die element 20 is received in the passage 16 of the body portion14, and the passages 18 and 22 of the body portion and die elementrespectively are aligned. The wire to be coated of course passesupwardly through the vertical passages 24a and 24b in the die element.

The die element 20 should be formed of a very finegrained material whichmay be given a very smooth bore in order to obtain a good finish on thecoating. Preferably, the material selected should be non-wettable by themolten coating metal and have good wear resistance. Ceramic materialssuch as Diamondite and Refrax has been found commercially satisfactory.Carbides having the desired properties may also be used.

Disposed in the passage 16 of the body portion just above the dieelement 20 is the member indicated generally at 26. As is clearly shownin FIG. 1, this member includes a portion 26a of reduced diameter, whichin conjunction with the walls of the passage 16 forms the plenum chamber28. The reduced diameter portion 26a of the member 26 is provided with aplurality of radial apertures 30. In the embodiment shown, theseapertures are arranged substantially normal to the wire 10, but it is tobe understood that this arrangement is not limiting. That is, undercertain circumstances, the apertures should be arranged obliquely to thewire axis. In this connection, reference is made to US. Pat. No.3,060,889, in the name of Earle L. Knapp, which discloses a swirlimparting gas finishing nozzle which may be used in connection with thisinvention. The body portion 14 is provided with the passageway 32. Oneend of this passage communicates with the plenum chamber 28, and theother end, via the tube 34, is in communication with a supply offinishing gas.

Held against the top surface of the member 26 is the hold down cap 36.It will be seen that the circular hold down cap is held in place bymeans of the wedges 38 4 which pass under the hold down brackets 40 and42 of the body portion 14, and serves to prevent movement of the dieelement 20 and member 26 due to the wire movement therethrough.

Secured in any suitable manner to the bottom of the body portion 14 andin a position to communicate with the horizontal passage 18 is themolten metal supply trough 44. It will be understood that the entireassembly may be moved vertically in relation to the trough 44, or thedie element 20 may be adjusted vertically within the body portion 14, inorder to vary the level of the coating metal in the die.

In operation of the apparatus, the trough 44 will be continuouslysupplied with molten coating metal to the level indicated by the line46. The relationship between the level of molten metal in the trough 44and the top surface of the die element 20 is a very important facet ofthis invention, and will be discussed in more detail presently. Forpresent purposes, it will be understood that by maintaining the troughfull of molten metal to this level, the aligned passages 18 and 22 willin effect define a reservoir of molten coating metal through which thewire 10 is drawn. In other words, the horizontal passage 22 in effectdivides the die element 20 into an entrance die portion 48 havingpassage 24a and an exit die portion 50 having passage 24b between whichthe coating bath is maintained.

Thus the wire 10 with a properly prepared surface passes from theprotective atmosphere inside the tube 12, through the entrance die 48and into the bath of molten coating metal. It emerges from the coatingbath through the exit die 50, and is immediately contacted by thefinishing gas through the apertures 30. Actual commercial use of theinvention has established that optimum finishing requires anon-oxidizing gas such as hydrogen under a relatively low pressure. Sucha gas retards oxidation of the still molten coating metal and, undersome circumstances, has a slight effect upon coating weight.

It is preferred that the entrance passage 24a be up to 0.10" smaller indiameter than the exit die passage 24b. This prevents the coated wirefrom rubbing on the wall of the exit die. Thus, a uniform coating isWithdrawn around the strand periphery.

Exemplary exit die clearances over the base wire diameter are asfollows:

6 ga. (0.192 ctd. wire dia.) (0.188" base wire dia.)

use 0.215" exit die 9 ga. (0.148 ctd. wire dia.) (0.145" base wiredia.)-

use 0.169" exit die 12 /2 ga. (0.099 ctd. wire dia.) (0.095" base Wiredia.)use 0.118 exit die. 8

For practical purposes a clearance of .010" to .040" on the diameter,preferably .015" to .025" should be maintained. If the clearance is toosmall, the coating metal will not feed uniformly with sufficient volume.If the clearance is too large, excess metal will accumulate in globularform on the top surface of the exit die and around the strands, chokingoff the apertures 30.

The principle of applying 0.40 oz. minimum coating to a wire strandinvolves pulling enough liquid metal into the clearance space betweenthe strand surface and the exit die wall to form a controllablemeniscus. This is stabilized at a level near the top surface of the exitdie by a combination of Wire speed Which promotes metal pumping action,die clearance, smoothness of the exit die bore wall, length of the exitbore, hydrostatic head of the melt supply in respect to the top surfaceof the exit die, fluidity of the coating metal, the downward or lateralpressure of the finishing gas jets, and the rate of aluminum oxidationon the top surface of the meniscus.

For a Diamondite die material and using the above die clearances, theexit die bore length has been designed at for wear life. This length canbe more or less depending on die material. The length of the entrancedie is also This permits turning a worn die end for end to ream out to anext larger wire size.

The fluidity of the coating metal within the exit die is controlled bythe temperature of the passing strand and of the melt supply. Wiretemperatures of 1240 to 1270 F. are preferred. Lower values will causean increase in wire coating weight. Higher temperatures tend to formexcessive oxide crusts on the top side of the exit die. Wiretemperatures in the order of 1350 F. and higher will cause the coatingweight to be less. These figures refer to high purity aluminum thatmelts at about 1220 F. Undoubtedly, both temperature ranges would shiftin relation to the melting point of coating metal.

The temperature of the aluminum coating metal bath is not highlycritical. A temperature between 1250 F. and 1300 F. has been foundadequate. The preferred range is on the order of 1270 F. to 1280 F., butit is believed that satisfactory results may -be achieved with atemperature ranging from the 1220 F. melting point of aluminum to asmuch as 1500 F.

Rather surprisingly, wire speed has been found to have very littleeffect on coating weights. While the evidence is not clear, it appearsthat faster wire speeds bring about a smoother and more brilliantsurface to the coating applied. Thus, commercial operations in excess of50 feet per minute are to be preferred.

By marked contrast to the above factors, applicant has found that therelative levels of aluminum in the coating bath and the top surface ofthe exit die can be utilized to control coating weights over a ratherwide range.

It will be understood that the relative levels of the coating metal aredetermined under static conditions preliminary to feeding the wire. Thedesired coating weight dictates the height at which the entire coatingassembly is positioned relative to the top surface of the metal in thetrough (line 46 in FIG. 1).

The following examples show the eifect of metal level on coating weight:

1 Metal level of aluminum supply in respect to top surface of exit die 2Coating weights for wire temperatures in the order of 1,240 to 1,270 F.Finish gas jets 30 with the strand except as noted.

3 Finish gas jets 90 to wire and impinging tangentially.

The coating device herein described is capable of applying high purityaluminum coating (99.75% purity in the outer coating layer) in theweight ranges of 0.30 oz. class (0.20 oz. minimum), 0.50 oz. class (0.40oz. minimum), and 0.80 oz. class (0.70 oz. minimum).

It will be seen from the foregoing exemplary data that the level ofmolten coating metal in the supply may vary from a height approximatelybelow the level of the top surface of the exit die to a heightapproximately /8" above the level of the top surface of the exit die. Asis well known in the art, molten aluminum around a rising coated wirestrand will form a concave meniscus. Therefore, even though theindicated static level of molten metal is above the top surface of theexit die, the meniscus is kept from expanding over the top surface ofthe die by controlling the impingement of the finishing gas. Under nocircumstances can the molten coating metal be permitted to flow over thetop surface of the exit die.

It has now been discovered that if the level of molten metal in thecoating bath is substantially equal to the level of the upper surface ofthe exit die (i.e. from about plus Ms" to about minus good, uniformaluminum coatings can be obtained in a weight up to about .80 ounce persquare foot of wire surface. As the static level of molten metal in thesupply is progressively adjusted to a lower height, coating weights arecorrespondingly decreased down to a weight of about .20 ounce per squarefoot of wire surface.

It is believed that the foregoing constitutes a full and completedisclosure of this invention. It will be understood that no limitationsare to be inferred or implied except as specifically set forth in theclaims which follow.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In a wire coating method including the steps of thoroughly cleaningthe surface of said wire, raising the temperature of said wire surface,and passing said wire through a bath of molten coating metal so that itemerges in a vertical path, the improved method comprising the steps ofproviding an exit die having an aperture at least partially submerged insaid bath, said aperture having a clearance over said wire of about .010inch to about .040 inch on the diameter, withdrawing said wire throughsaid aperture without further contacting said molten coating metal, andcontrolling the level of molten metal in said bath with respect to thetop surface of said die so that said level is substantially at said topsurface of said die whereby to control the thickness of coating metalapplied to said wire.

2. In a wire coating method including the steps of thoroughly cleaningthe surface of said wire, raising the temperature of said wire surface,and passing said wire through a bath of molten coating metal so that itemerges in a vertical path of travel through an exit die having aclearance over said wire of about .010 inch to about .040 inch on thediameter, and without further contacting said molten coating metal; theimproved method of controlling coating thickness comprising the step ofmaintaining the level of molten coating metal in the bath not more thanabout 7 inch below and not more than about A; inch above the top surfaceof said die.

3. The method of applying a molten metallic coating to a ferrous wirecomprising the steps of:

(a) thoroughly cleaning the surface of said wire;

(b) raising the temperature of said wire surface;

(c) passing said wire into a bath of molten coating metal, withdrawingsaid wire from said bath of molten metal through an exit die having aclearance of about .010 to about .040 inch on the diameter of said wirein a vertical path without further contacting said molten metal; and

(d) controlling the level of molten metal in said bath with respect tothe top surface of said exit die so that said level is not more thanabout inch below and not more than about Ms inch above, said top surfacewhereby to control the thickness of coating metal applied to said wire.

4. The method claimed in claim 3 wherein said molten metalliccoatingmetal is aluminum.

5. The method claimed in claim 4 wherein said bath of molten coatingmetal is maintained at a temperature in the range of l250-1300 F.

6. The method claimed in claim 5 wherein the said wire surface is raisedto a temperature substantially equal to or slightly less than thetemperature of said metallic coating bath.

7. The method claimed in claim 6 wherein said step of raising thetemperature of said wire surface is accomplished in a reducingatmosphere.

8. The method claimed in claim 7 including the step of providing aprotective atmosphere for said wire after leaving said reducingatmosphere and prior to entering said molten coating bath.

9. The method claimed in claim 1 for production of coatings of at leastabout .50 ounce per square foot of wire surface wherein said level ofmolten metal in said bath is at least substantially at said top surfaceof said die and not more than about /8 inch above said top surface ofsaid die.

FOREIGN PATENTS 972,527 10/1964 Great Britain. 1,030,967 5/1966 GreatBritain.

References Cited UNITED STATES PATENTS 5 ALFRED L. LEAVI'I'I, PrimaryExaminer Girvin J. R. BATTEN, JR., Assistant Examiner Herman.

Whitfield. US. Cl. X.R. KnaPP X 10 117-114, 128; 118-63, 405 Baessler eta1.

Knapp.

